The invention relates to a circuit arrangement for the compensation of temperature-dependent and temperature-independent drift and for the compensation of the sensitivity of a capacitive sensor, which comprises two measuring capacitors whose capacitances are connected to at least one oscillator which outputs pulses having a duration which is proportional to the capacitance. The pulses arising from the respective capacitances appear in an alternating fashion. The invention also comprises a circuit for generating reference pulses from the pulses arising from the respective capacitances, the duration of the reference pulses being shorter than the duration of the pulses from the respective capacitances. The invention further comprises a pulse duration demodulator which is associated with a respective capacitance and which forms, from the pulses applied, a d.c. signal which corresponds to the relevant reciprocal value of a capacitance in that during a first period t.sub.ref which depends on the duration of a reference pulse a first reference signal is up-slope integrated and during a second period, after expiration of the reference pulse, a second reference signal is down-slope integrated. The invention also comprises an output subtraction member which forms the difference between the two output signals of the pulse duration demodulators.
An evaluation circuit for a capacitive sensor of this kind is described in German patent application No. P 35 28 416.1 (PHD 85-115); evaluation circuits of this kind are used, for example for determining the differential pressure, measured by a capacitive differential pressure measuring device, by evaluating the pressure-dependent capacitance variation of two measuring capacitors included in the differential pressure sensor. This evaluation circuit, however, exhibits a sensor-dependent drift of the zero point and the sensitivity in dependence on temperature variations.
In principle it is known (DE-OS No. 33 40 834) to realize compensation of zero point and sensitivity in a capacitive differential pressure sensor by converting the sensor capacitances C.sub.1 and C.sub.2 into a.c. voltages by means of integrators, the amplitude then being proportional to 1/C.sub.1 and 1/C.sub.2, respectively. In this known circuit arrangement, the temperature-dependent sensitivity of the sensor is compensated for by summing these a.c. voltages with a reference a.c. voltage which originates from a further integrator and which is formed by feeding back this sum signal to an oscillator which itself drives the integrators. The zero point can be corrected by summing and subtraction of the integrator voltages. This known circuit arrangement, however, operates only if the sensor information is present in the amplitude of the electric signal. However, when the sensor information is encoded in the pulse width or the frequency, as is the case in the evaluation circuit of the present kind, this known method of compensation cannot be applied due to the completely different way of operation of the relevant circuits.